If you are sure it is the presuure switch best thing to do to test it is to wrap a tissue bandage around its top and wait to see it if gets soiled. If it is going to leak it will leak through its plastic cap usually.
Only other way I can think of is to remove the scuttle, plenum, then blank off or bypass all pipes and then run it while you can see the full top end. Migs are notorious for leaks and I have yet to find another car that has so many freekin pipes, joints and possible places for leaks to develop that are hidden from view.
Did it to mine and now it is leak free (for now at least I suppose).
GudLuk
OK - definitely not the pressure switch. Have had it in the air today at the garage. It had a good rinse down, drying off and then left running to see what's occurin'.
Not the switch, Not the cam covers. BUT potential trace from what appears to be the drivers side camshaft seal (between cambelt cover back plate and cam cover area - trace of a drip line visible) which is leaking down behind the alternator. ALSO - but not yet certain a possible weaping seal between sump cover housing and what I assume is the engine block above it - just behind where the pressure switch sits.
Couple of questions if I may:
1. Any special Vx tools required to change the camshaft seals ? the garage I use haven't done one before on this engine.

2. Is it any more tricky than a belt change ? They have done quite a few of these though.

3. Cambelt etc. replaced 2.5k miles ago - will any of this need changing again if it's disturbed.

4. Anything else worth changing whilst this lot is out ?

If after this is done I've still got the sump block seal issue - I'll be back 8-).
Cheers
Ian.